• Clinical science



Legionellosis is an infection caused by Legionella pneumophila, a gram-negative rod that thrives in warm aqueous environments such as drinking-water systems, hot tubs, and air-conditioning units. Transmission occurs by inhaling contaminated, aerosolized water droplets. Legionellosis is a common nosocomial infection and outbreaks are typical. Notable risk factors include smoking, chronic lung disease, advanced age, and immunosuppressive conditions. There are two forms of legionellosis: Legionnaires' disease and Pontiac fever. Patients with Legionnaires' disease present with atypical pneumonia (shortness of breath, cough), commonly in combination with various other symptoms, including gastrointestinal (e.g., diarrhea) and neurological (e.g., confusion). Laboratory abnormalities are common, especially hyponatremia. Pontiac fever is a milder, self-limiting, flu-like illness. A urine antigen test is used to confirm infection with L. pneumophila. Fluoroquinolones are the treatment of choice for Legionnaires' disease. In the US, legionellosis is a notifiable disease and steps should be taken to eliminate contaminated sources and to prevent future outbreaks.


  • Frequency: : occurs rarely in infants, almost solely in adults (of any age) and typically in outbreaks
  • High-risk groups [1][2]

Epidemiological data refers to the US, unless otherwise specified.


Causative organism

  • Legionella pneumophila (gram-negative, obligate aerobic, facultative intracellular rod) causes over 90% of Legionnaires' disease outbreaks.
  • The optimal water temperature for Legionella is 25–42°C (77–108°F). [3][4]
  • Pontiac fever is generally due to lesser-known types of Legionella.

Path of infection

  • Inhalation of contaminated aerosols
    • Cold and hot water systems (e.g., those found in hotels, hospitals, and retirement homes)
    • Whirlpools/hot tubs, swimming pools, showers
    • Air-conditioning systems with contaminated condensed water
  • Person-to-person transmission is uncommon.

Since transmission from person to person is uncommon, isolation is unnecessary.

Locations at particular risk of outbreak

  • Nursing homes
  • Hospitals
  • Confined travel accommodations (e.g., cruise ships, hotels, resorts)


Clinical features

Legionnaires' disease

Legionellosis should always be considered in patients with signs of atypical pneumonia and diarrhea in combination with possible exposure (e.g., cruise ship travel, use of a whirlpool).

Pontiac fever

  • Incubation period: 1–3 days
  • Mild, self-limiting course of legionellosis without pneumonia.
  • Flu-like symptoms (e.g. fever, headache, and muscle ache)



See “Diagnosis of pneumonia”.

Laboratory findings

The legionaries drew their iron swords, donned their silver helmets, and jumped off the ship to burn the town to coal: legionellosis, iron buffered medium, silver stain, history of cruise ship travel, charcoal yeast extract agar.

Imaging [9]

  • Chest x-ray: Diffuse reticular opacities are commonly seen (especially in atypical pneumonia).
  • Chest CT
    • Bilateral or unilateral consolidative changes and/or ground-glass opacities
    • Air bronchograms may also be seen.


Legionnaires' disease should be treated early with antibiotics, especially because of its high mortality rate of 10%. [10]

In any case of atypical pneumonia, antibiotic treatment needs to cover Legionella.


  • Legionellosis is a notifiable disease.
  • Course of action when contaminated water sources are detected in medical facilities
    • Contaminated water systems should be disinfected .
    • Use terminal tap water filters, especially for high-risk patients (e.g., immunocompromised or the elderly).